Fix Xcode 11 build (facebook#25146)

Summary:
Fixes build in Xcode 11 beta, the signature for `__unused` was changed. This adds a new check for the new style.

[iOS] [Fixed] - Xcode 11 beta build
Pull Request resolved: facebook#25146

Differential Revision: D15628404

Pulled By: cpojer

fbshipit-source-id: 781a188a0e1562a3316fbe62920b12b03a44e4a7
(cherry picked from commit 46c7ada)

React/Base/RCTModuleMethod.mm

@@ -83,8 +83,9 @@ static BOOL RCTParseSelectorPart(const char **input, NSMutableString *selector)
 
 static BOOL RCTParseUnused(const char **input)
 {
-  return RCTReadString(input, "__unused") ||
-         RCTReadString(input, "__attribute__((unused))");
+  return RCTReadString(input, "__attribute__((unused))") ||
+         RCTReadString(input, "__attribute__((__unused__))") ||
+         RCTReadString(input, "__unused");
 }
 
 static RCTNullability RCTParseNullability(const char **input)